Multiplex facsimile system



April 15, 1958 M. ARTzT 2,831,056

MULTIPLEX FACSIMILE SYSTEM Original Filed May 13, 1950 4 Sheets-Sheet 1inf/@Az April 15, 1958 M. ARTZT MULTIPLEX FACSIMILE SYSTEM OriginalFiled May 13, 1950 4 Sheets-Sheet 2 INVENTOR A TORNEY April 15, 195s M.ARTZT MULTIPLEX FACSIMILE SYSTEM Original Filed May 13, 1950 4Sheets-Sheet 3 INVENTOR BY "fr LRA: 4 f7 j April 15, 1958 M. ARTzTMULTIPLEX FACSIMILE SYSTEM Original Filed May 13, 1950 4 Sheets-Sheenl 4.ww 1N ww. \\m. wlllllllllih QS Q. i lli @n \\l/ Illu Mmmm NX, @Awww l Is NQ Sil nited Mnrrrrrnx racsnvmn svsrnM Maurice Artzt, Princeton, N.J., assigner to the United States of America as represented by theSecretary of the Army 3 Claims. (Cl. 173-11) This invention bears onfacsimile systems, namely, but not exclusively, on the aspect of thissubject as it concerns multiplex transmission and reception of subjectmatter in facsimile. This is a division of my application S. N. 161,887,filed May 13, 1950, now Patent No. 2,769,856, issued November 6, 1956.

Transmission of facsimile signals at a high speed presents specialrecording problems. Photographic recordino may be used but it hascertain inherent disadvantages. For chemical recording, recorder partsmust move at such high speed that design and production of the recorderis almost impossible.- By this invention means are provided forgenerating more than one series of facsimile signals upon scanning asingle piece of subject matter. Each series is sent over a separatechannel. The sum of the bandwidths of all channels, including guardbands and unused lchannel portions, is about the same as the bandwidthof the single channel which would be used to transmit the subject matterat the same speed. The recording speed per channel when the invention isused permits good mechanical design of the recorder. In a system usingthis invention a novel multiple helix recorder is provided having acommon printer bar. Means are present to prevent inter-channelinterference by limiting voltage surges in each printer helix.Electrical delay lines are used to compensate for mechanical orelectrical mismatching in the system. The delay lines are used aftersignal detection so that all delay lines become the same bandwidthregardless of carrier frequencies used in each channel. Mechanicalinaccuracies in the recorder are readily compensated for by the delaylines. Change of transmission speed without altering synchronism orphasing is possible with the invention.

The principal aim of the invention is to obtain highl speed facsimiletransmission and overcome disadvantages associated with high speed priorart transmission.

Another aim of the invention is the realization of the advantagespointed out above.

Other objects will be seen by reading this specication which refers tothe drawings in which:

Figure 1 is a schematic showing of a recorder embodying-the invention;

Figure 2 is a developed View of the recording of Figure 1;v n

Figure 3 is a schematic showing of a preferred type of recorderembodying the invention;

Figure 4 is a developed view of the recording of Figure l;

Figure 5 is a curve showing a total bandwidth required for a high speedfacsimile operation by prior art methods; Figure 6 shows the totalspectrum and manner of its use for high speed facsimile operation inaccordance with the invention;

Figure 7 is a schematic showing of a system employing 'the recorder ofFigure 3;

`vfigure, 8 is a diagram of a signal amplifier embodying automatic gaincontrol means;

rates Patent 0 The symbol fk designates the keying frequency.v This is2,831,056 Ice Patented Apr. 15,1958

Figure 9 shows, schematically, the electrical circuit of the recorderand an associated variable delay line;

Figure 10 is a transverse sectional view of a twochannel transmitterscanner embodying the invention, the section being taken on line1li-lil; and

Figure 11 is a longitudinal sectional view of the scanner of Figure 10,the section being taken on line 11-11.

Figures 1 and 3 of the drawings show recorders for use in a system ofthe invention. The recorder of Fig. 1 is designated by the referencecharacter 10 and includes a drum 12 having a single helix 14 securedthereon. A recording web 16 passes over the drum and is moved by asuitable means (not shown) inthe direction of the arrow. The recorder ofFig. 1 is to be employed with the four transmission channels and fourprinter bars 13 to 21 as indicated. The drum is carried by a shaft 22which is driven for rotation by any suitable means. Fig. 2 shows the web16 with a recording pattern developed thereon. Fig. 2 shows the way inwhich the scanning lines are produced on the web 16 to form copy havinginterlaced lines.

The printer bars 18 to 21 must be exactly parallel to each other and tothe drum axis. The spacing between them must be very exact for the linesto interlace correctly as shown in Fig. 2. As all of the printer barsare insulated from each other to carry the separate printing currents,they must be spaced apart. Optimum spacing is 1/3 of an inch. At leastone inch of copy must pass through the recorder before the completefour-line linterlace scanning pattern is obtained over the entiresurface. Loss of time is thus encountered at the start of each subject.v

Fig. 3 shows a preferred arrangement for arecorder. The recording web 16is moving inthe direction of the arrow tangent to a drum 24. Fourhelices 26 to 29 are equally spaced around the periphery of the drum.Each helix is insulated from the drum and has a slip ring connection asindicated on Fig. 7. The drum 24 is carried and driven by a shaft 32. Asingle printer bar 34 is disposed parallel to the axis of the drum 24.The printer bar 34 is common to all four channels. Accurate spacing ofthe helices on the drum 24 -is desirable. An advantage of thearrangement of Fig. 3 is that small errors in angular spacing can becorrected by variable time delays inserted in the signal channels.

between 27 and 2d is 90, the angle between 28 and 29 is slightly lessthan and the angle between 29 and 26 is slightly greater. On Fig. 4scanning lines 26, 27 and 28 would be printed in correctpositions. Line29 would be printed slightly in advance of its true position. Signals tohelix 29 could be delayed the correct amount to makev high speedoperation. In order to carry the D. C. com-- ponent of the video signal,they are transmitted as a,

modulation on a carrier. This carrier must `be higher than the highestvideo frequency encountered. Even with single side band amplitudemodulation, a use ratio much higher than 0.5 is seldom obtained. Forexample, a 192 kc. channel will accommodate a recording speed of 667 sq.in. per minute with lines per inch detail. Fig. 5 shows the unused lowerportion of the band in simplex operation. In the given example, this is72 kc. Fig.

6 shows how this unused lower portion of the band can.`

be divided into guard bands to obtain the same use ratio for a.four-channel multiplex system of this invention.

96 kc. in the given example. fc is the carrier. This is As an example,itwill be assumed that the angle between 26 and 27 and 3 168 kc. Eachchannel of Fig. 6 uses equivalent symbols.

Fig. 7 shows schematically a four-channel multiplex. Each channel isone-quarter of the width of the full band required to transmit the sameimage in the same time over Aa single channel. It will be understood bythose skilled in the art that a different number of channels may beemployed. The transmitter scanner 36 may be constructed as shown inFigs. l and 1l. These latter figures show an illustrative two-channelscanner and will be described later. The scanner 36 has a common lightsource 38 with 4 optical paths and 4 phototubes 41 to 44. The outputs ofthe phototubes are used to amplitude modulate the four spaced carrierfrequencies f1 to f4. After ltering out the upper side band of eachchannel, the four single side band signals are added and fed to thetransmission line 48. The modulators 49 to 52 may be of any desiredtype.

Fig. 6 shows the relationship of the channel signals in a spectrum. Thefrequencies f1 to f4 may be 42, 90, 138 and 186 he. The modulators maybe connected to give maximum amplitude of signal on black. The bandwidth of the single side band is 24 kc. with a total band width of 48kc. per channel. The guard band between channels of the system is 14 kc.This 14 kc. is ample for functioning of the filters for channel signalseparation. These values are given by way of example.

At the receiving end the four signals are separated by filters 53 to 56.Fig. 6 of the drawing with the given description amply indicates thenature of these filters and they may be of any known type. The output ofeach filter is fed to an amplilier and detector combination. Theamplifier portion is equipped with an automatic gain control (A. G. C.).These amplier-detectors are designated 57 to 68 on Fig. 7. The detectedoutput for each channel is fed to a printer amplifier through avaria-ble delay line. The latter are designated 61 to 64 on Fig. 7.' Theprinter amplifiers are designated 66 to 69. Fig. 8 shows one of theamplifier-detectors 57 to 60. Fig. 9 shows one of the variable delaylines 61 to 64, and, also, one of the ampliiiers 66 to 69. Eachamplifier 66 to 69 is connected to one of the helices 26 to 29 on thedrum 24 of the recorder (Figs. 3 and 7) by way of slip rings 71 to 74.These slip rings may -be radially disposed on an insulating plate in awellknown manner rather than as shown on Fig. 7 for convenience.Suitable jumpers or connectors are provided between each slip ring andits associated helix.

It will be noticed that the variable delay line has been placed in thecircuit after detection rather than before. This has been done so thatall lines may have the same hat delay band of zero to fk, and anychanging of channels or lters that may be found necessary in servicewill not affect the delay correcting network.

The speed of recording can be cut to one-half by using only the channelsof phototubes 41 and 43 and cutting the paper feed to one-half the fullvalue. The drum speed remains the same as for four-channel operation.Under these condtions, a two-line interlace scanning pattern is obtainedand a 96 kc. bandwidth used. Any one channel can be used alone withpaper feed reduced ot 1A full rate, and a single 'channel of 48 kc.bandwidth is required. Changing speed between these three values of 12or 4 times the slowest speed is thus accomplished by a gear shift on thepaper feed and by switching in or out the necessary channels. Changespeed gear drives are -by now well known and need not be shown. Drumspeed remains constant so synchronizing and phasing are unaffected byspeed changing.

The illustrative two-channel scanner of Figs. 10 and 11 was mentionedabove in connection with the fourchannel scanner 36 of Fig. 7. Referringto Figs. 10 and l1, the scanner comprises a carriage 91 equipped withrollers 92. The latter roll on a trackway provided by tubular rails 93and 94. The carriage is moved along the rails to provide one componentof scanning action ,4. by means of a cord 96 and a driven reel 97. Reeldrives are known in the art which coordinate the rotational speed of thescanning head 98 with its desired rate of axial travel. For example, aworm pinion (not shown) on the drive shaft 101 for the scanner may meshwith a worm wheel (not shown) on the reel shaft 102. The shaft 101 iscoupled to a motor (not shown) which is preferably of the constant speedtype. A synchronous motor may be used. Any known or desiredsynchronizing system may be employed to keep the recorder driven in stepwith the scanning head 98. Sync and phasing signals may be obtained by acommutator (not shown). The gap in the subject copy may also be used toprovide a sync and phasing signal. A lbracket 106 on the carriage 91carries a ball bearing assembly 108 which supports one end of the head98. The other end is supported in a suitable bearing and extensibledrive connection (neither shown).

A stationary housing or frame 110 provides support for a transparentcylindrical member 112 which surrounds the head 98. Subject copy iswrapped around the member 112 from one support 114 to the other 116. Thegap between supports may provide the sync and phasing signal asindicated above. In addition to providing support for the cylindricalcopy holder 112, the housing or frame 110 also supports the motor andthe additional bearing, mentioned above, for the shaft 101 (neithershown).

The scanning head 98 comprises a body member 121 which may be cast orotherwise formed of aluminum, for example. The body 121 is provided withcylindrical bores or apertures 122 and 123. These bores accommodatephototubes 124 and 125 or similar devices to generate image signals asthe subject matter on the copy holder 112 is scanned. Slip rings 126 to129 have suitable connections to the phototubes and cooperate withbrushes mounted on a carriage bracket 131. The use of .a carbon brush ona silver slip ring does not introduce noise.

The body member 121 has an extended portion 133 with a chamber 134 toaccommodate a lamp 136. A slip ring 138 for the lamp is threaded ontothe extension 133. An insulating collar 141 is also threaded onto theextension and carries a second slip ring 142. A rod 146 is clamped in abore in the member 121 by means of a set screw 148. Loosening of thescrew permits the rod 146 to be rotated and be moved axially. The rod146 has a flat mirror surface 151 at its end. Another rod 152 is alsoseated in the bore land is clamped by a set screw 153. This second rodis provided with a mirror end 154. Light from the exciting lamp 136 goesthrough a passage in the body 121, impinges on the mirror 151 and isreliected from the mirror 154 through a lens system 156 onto the subjectcopy as a spot image. Light from the spot on the copy goes through thepassage 158, a Lucite rod 161 serving as a cylindrical lens to theactive element of the phototube 125. This structure is duplicated forthe phototube 124 but displaced 180. It will be understood that detailsof the structure just described may be employed in the schematicarrangement indicated in Fig. 7.

Fig. 8 showing details of the gain control amplifier 57 of Fig. 7 willnow be described in detail. The units 58 to 60 are similar. Input to theamplifier 57 is applied to terminals 181 and 182. A volume or gaincontrol 184 serves to couple the input to the control gn'd 186 of anamplifier tube 188. The tube 188 and a tube 189 act as amplifiers, withthe necessary grid and plate resistors and coupling capacitors of suchvalues that amplification is linear up to about 300 kc. A power supply(not shown) is connected as indicated to the terminals 192, 193 and 194.The tubes 188, 189 and a phase inverter tube 198 are connected betweenpositive supply terminals of the power supply and ground. The push-pullsignal from the outputV of the phase inverter tube 198 is rectified by a'full-wave rectifier 201. The output of the rectifier or detector 201appears at terminals 204 and 206 and is to be applied to the terminals208 and 209 in Fig. 9. A portion of the signal from the cathode 211 ofthe tube 198 is rectified by a tube 212 serving as a cathode followertype of rectifier. The threshold setting is adjustable by apotentiometer 214. The cathode follower action of the tube 212 charges acondenser 216 through a very low impedance, and full voltage can bedeveloped across the condenser 216 for signals of extremely shortduration. Bleeder resistor 218 across the condenser 216 is of a highenough value that the fall in voltage across the condenser 216 is only aVery few percent per scanning line of the subject copy. A tube 221 actsas an amplifier and reversing stage for the A. G. C. voltage developedacross the condenser 216. The plate resistor 223 of the tube 221furnishes the control bias for the amplifier tubes 188 and 189. Thecondenser 216 is not used for A. G. C. signal storage but is made onlylarge enough to prevent oscillation.

With the A. G. C. just described the ratio of charge to discharge can bemade as high as required, and the range of gain or voltage from nocontrol to full bias is extremely narrow. With all of the channels ofthe multiplex system described herein controlled in this fashion, onechannel input can suddenly be increased with respect to the otherwithout appreciable marring of the received copy. Readjustment is alsosufficiently rapid.

In addition to have an A. G. C. system, the second requirement of therecording system is that the output printer stage be a true constantcurrent device with the values of current for any shade of gray the samefor all output stages. If this condition is not met, printer currents-for the various scanning lines will be sufficiently different as barpressure varies to give a noticeable nonuniformity of scanning linepattern. The chemical action of recording has a linear curve of densityvs. printer current (log scale), but the impedance of the recordingcontact varies over wide limits, decreasing as current increases. Thiscontact resistance also is affected to some extent by bar pressure, soslight mechanical irregularities in pressure would change the printingcurrent if the printer stage were not of a constant current type.

A printer amplifier that meets those requirements is shown in Fig. 9.The printer load, represented by a helix and the printer bar, isconnected in the cathode circuit of a tube 228 to get the properpolarity of input to helix and bar for recording on the front of thepaper. The signal polarity is indicated on the drawing. Three 807 typetubes in parallel can be used for the tube 228. When the tube 228 isconnected as shown its screen must be supplied by a separate floating250 volt power supply to maintain a true constant current action of theoutput stage. This supply (not shown) is connected between terminals 232and 233. The power supply (not shown) for the tube 228 is connectedbetween the terminals 234, or ground, and 236. The negative terminal ofthe power supply or an additional power supply (not shown) is connectedto terminal 238. This serves the tube 240. The peak value of the outputcurrent is regulated by the value of the cathode reistor 241. In orderthat all printer stages may be set to the same peak black current, thisresistor is tapped rather than made smoothly variable. When the peakcurrent of each channel is adjusted by the black trimmer 246 to adesired current value for the tap 248, all other taps will accuratelygive the specified currents over Widely varying printer bar and helixcontact resistances. This was found to hold true with variations in theheight of the two helices as high as .O05 inch so manufacturingtolerances on the multihelix drum are not excessively severe. I

Due to the degenerative action of the resistor 241 being in plate,screen and grid circuits, this method of current control does not alterthe value of signal voltage necessary across a resistor 251 to bias theoutput stage to cut off for white. The rectified signal input shown isheld constant by the A. G. C., and, therefore, the volume controlpotentiometer 253 can be set Afor maximum white signal to just drivetube 228 to cutoff, and this white condition is then held for allsettings of 241. The half tone scale is therefore the same for allvalues of output current, and for any particular recording condition thedifferent channels need only be set to the same tap of 241 to 'have auniform scanning line pattern. The black controls can be ganged ifdesired.

The plate voltage of the tube 240 will vary with signal,

for the voltage across the printer is added to the power supplyconnected to terminal 238. However, the screen 273 of the tube 240 isheld fixed so its plate current through the resistor 251 is constanteven though the current and impedance of the printer circuit varies. Thevoltage developed across the resistor 251 thus depends only on signalinput to the tube 240 and a constant drive to the grid of the tube 228is assured.

A feature of the printer amplifier of Fig. 9 is the addition of a diode276 to limit the voltage developed across the printer. Interactionbetween the printers of the system takes place at the end of eachscanning line of each helix. The reason for this is that at the end ofeach helix the printer circuit is momentarily broken before the enteringedge of that same helix starts the succeeding scanning line. The voltageacross the printer then momentarily rises to the full value of the platesupply across terminals 233 and 234. In the example, this is 400 volts.Leakage conductance across the wet paper to the other helix causes asmall spot of color to be printed. Ilf this voltage surge is limited toabout 200 volts no interaction takes place, and this limiting isaccomplished by the diode 276 connected between a 200 volt tap on ableeder 281 and 282 and the gride of the tube 22S. The grid of the tube228 is prevented from rising above +200 volts, and the cathode thereforeis limited to this same surge voltage. No effect is found on theprinting, for the maximum printer current is developed with a littleover 100 volts to the printer circuit.

The delay lines 6l to 64- are placed after the detector of the recordingamplifier so that all delay lines handle the same frequency spectrum,and where low enough cutoff frequencies can be used to make the steps ofthe required amount. With the highest keying frequency per channel of 26kc., lines with a cutoff Ifrequency of 39 kc. or higher will giveuniform delay over the video band. For 5 microseconds per stage, thecutoff frequency should be 63 kc., so steps of 5 microseconds can beused with no appreciable distortion. With single channel operation athigh speed using a keying frequency of 96 kc., a total delay variationof 5 microseconds over the entire band is the maximum allowable. Withmultiplex operation as taught herein, microseconds is permissible or 20microseconds per channel. This is a net improvement of 16 to l.

The tap switch used to vary the delay is shown in Fig. 9 connected tothe volume control 253. This control must be of high resistance to avoidloading the filter at intermediate taps and disturbing its termination.The filter has some loss, and volume is decreased as the delay isincreased. This is corrected by inserting the correct value ofresistance in series with each low delay tap to decrease the voltageacross 253 to that obtained when the full line is switched in. Changingdelay therefore does not affect the White volume setting, but onlyshifts phase.

1t should be pointed out that these are tlat delay lines and are usedfor correction of mechanical inaccuracies and/ or differences in averagetotal delay between channels. They are not intended to correct fordifferential delays within a channel, so the previous delay toleranceson the channels themselves must still be met by the transmission medium.

\What is-claimed is: l i f l. Facsimile apparatus comprising'- aylindrical light transmittingfcopy holding member for supporting amessage-sheet, a rotary scanning device positioned internally of saidcopy holder, with said scanning device being adapted to rotate .upon anaxis substantially coinciding with the axis ofthe copy holder, means toprovide relative movement between said copy holding member and saidrotary scanning device, a light source, a photo-tube, means providing alight path from said light source to said copy Ifhijldrf'a lightreflecting member in said light path comprising a cylindrical rod havinga reflector provided on its end, said scanning device having a borewithin which said rod is received, said rod being rotatable and movablelongitudinally in said bore to provide adjustment of said light path.

2. Facsimile apparatus comprising a cylindrical light transmitting copyholding member for supporting a message sheet, a rotary scanning devicepositioned internally of said copy holder with said scanning devicebeing adapted torotate upon an axis substantially coinciding with theaxis of the copy holder, means to provide relative movement between saidcopy holding member 'and said rotary scanning device, a light source, aphoto-tube, means pr`oviding a light path from said light source to saidcopy holder, a light reflecting member in 'said light-'path corn-Vprising a cylindrical rod, the end ofsaid rodk presenting an angularlydisposed light reflecting surface, said scanning device having a boreWithin which said rod is received, said rod being rotatable and movablelongitudinally in said bore to provide adjustment of said light path.

, Fas-.cimilepapparatus comprising a cylndricallight transmittingcopyholding member for supporting a message sheet, a rotary scanningdevice, positioned internallyy of said copy holder With said,scanningfdevice being adapted to rotate upon an axis substantially coinciding-'with'the axis of said copy holder, means toprovide relativemovement between said copyholding member and said rotary scanningdevice, said rotary scanning device comprising a generally cylindricalbody, a recess in saidbody and coaxial therewith to accommodate alightsource, aplurality of angularly spaced recesses yin said body, aphototube in each recess, a bore in said body providing a light path, asecond bore intersecting said first bore, a pair of cylindrical rods insaid second named bore, the end of each rod presenting an angularlydisposed light reecting surface, said rods being rotatablel and movablelongitudinally in said boreto provide adjustment of said, light path,and

an aperture in said second named bore to direct light to said copyholder.

References Cited, in the tile of this patent UNITED STATES PATENTSv

